1. Field of the Invention
The present invention relates generally to microstructures, such as air-clad fibers, and in particular to an air-clad embodiment for a 3-level fiber laser.
2. Technical Background
It is known that for efficient coupling into a single-mode fiber, a pump source doped fiber should also be single-mode. However, multimode broad-area diode laser remains the most power efficient and least expensive pump source for optically pumping the doped fiber. Recent progress in semiconductor laser technology has led to creation of broad-area laser diodes with output powers of up to 16 W. Devices 100 μm wide with a slow-axis numerical aperture (NA) of less than 0.1 and output power of 4 Watts at 920 and 980 nm are now passing qualification testing for telecommunication applications. With proper coupling optics 70, the beam of such a laser diode can be focused into a spot as small as 30×5 μm with an NA of less than 0.35 in both transverse directions. The optical power density in such a spot is ˜1.3 MW/cm2, which should be high enough to achieve transparency in 3-level laser systems.
Brightness conversion is also known. One approach for brightness conversion utilizes inexpensive high-power broad-area pump lasers optically pumping cladding-pumped, or double-clad fiber designs to provide a high powered optical pump fiber laser. The advantages of cladding-pumped fiber lasers are well known. Such a device effectively serves as a brightness converter, converting a significant part of the multimode pump light into a single-mode output at a longer wavelength.
Cladding pumping can be employed to build a high-power single-mode fiber pump laser. A source based on the pure three-level 978 nm Yb+3 transition has long been suggested as a pump for known erbium doped fiber amplifiers (EDFAs) because this wavelength is close to the desired pumping wavelength of 980 nm. However, the cladding-pumped technique has been determined in practice to be ineffective for pumping pure three-level fiber lasers, such as the 980 nm transition of ytterbium, because of various fiber laser design parameters that have to be satisfied. A fiber amplifier is just a laser without a cavity for reflecting light back and forth that is required for lasing.
Practical double-clad amplifiers and lasers have been mostly limited to 4-level systems. Double-clad fiber lasers offer better performance for four-level lasing (where the lasing occurs in a transition between two excited states) than for the three-level one (where the lasing transition is between the excited and the ground state). For example, for the rare-earth ion, Ytterbium (Yb), the three-level transition is at 978 nm and competing higher-gain four-level transition is at about 1030-1100 nm.
In a double-clad laser, an outer cladding confines the pump light from a primary pump source in a large cross-sectional area multimode inner cladding. The much smaller cross-sectional area core is typically doped with at least one rare-earth ion, for example, neodymium or ytterbium, to provide lasing capability in a single-mode output signal. Typically, a neodymium-doped or ytterbium-doped double-clad fiber is pumped with one or several high-power broad-area diode lasers (at 800 nm or 915 nm) to produce a single transverse mode output (at the neodymium four-level transition of 1060 nm or the ytterbium four level transition of 1030-1120 nm, respectively). Thus, conventional double-clad arrangements facilitate pumping of the fiber using a multimode first cladding for accepting and transferring pump energy to a core along the length of the device. The double-clad laser output can be used to pump a cascaded Raman laser to convert the wavelength to around 1480 nm, which is suitable for pumping erbium.
How much pump light can be coupled into a double-clad fiber inner cladding depends on the cladding size and NA. As is known, the “etendue” (numerical aperture multiplied by the aperture dimension or spot size) of the fiber should be equal to or greater than the etendue of the pump source for efficient coupling. The numerical aperture and spot size may be different in both axes so there may be an etendue in the x and y directions that must be maintained or exceeded.
Typically, a high numerical aperture NAclad, related to the difference in refractive index between the first and second cladding is desired. If there are two claddings instead of one, the index of the first cladding is nclad,1 and the index of the second cladding is nclad,2 such that NAclad=(nclad,12−nclad,22)1/2. In the well-known design, the first clad layer is made of glass and the second is made of plastic (fluorinated polymer) with a relatively low refractive index in order to increase the numerical aperture NAclad. Such plastic may not have the desired thermal stability for many applications, may delaminate from the first cladding, and may be susceptible to moisture damage.
In known double-clad host fibers, the laser cavity is formed by an input dielectric mirror which transmits the 920-nm pump band and reflects the desired 980-nm lasing band. For any input mirror of the fiber laser, it is a desire to reflect only the fundamental mode, at the laser wavelength, e.g., 978 nm, to form the input end of the optical cavity. A dielectric mirror at the end of the double-clad fiber or a weak fiber Bragg grating in the single-mode fiber, e.g., Corning® CS-980 fiber, coupled to the coupling end of the double-clad fiber serves as the output coupler for providing the output end of the cavity.
One of the primary technical challenges in a high power fiber laser is the formation of the input dielectric mirror across the multimode inner cladding of the double-clad fiber. Approaches include attaching a glass micro-sheet to the fiber endface or directly depositing a thin-film dielectric on the fiber endface, but both of these methods present their own technical hurdles.
A two-stage fiber laser has also been proposed as an alternate optical pump. This two-stage laser has an optical pump source to provide a pump light at a pump wavelength. A first waveguide portion which when optically pumped at the pump wavelength is capable of lasing with an emission at a lasing wavelength. The first waveguide portion exhibits multi-transverse-mode behavior at the lasing wavelength. A second waveguide portion exhibiting a substantially single transverse mode behavior at the lasing wavelength is optically coupled together with the first waveguide portion. An optical cavity is defined by a multimode grating on the first waveguide portion and a single-mode grating on the second waveguide portion and includes the first and second waveguide portions. The delta index or contrast index of the difference between the cladding refractive index and the multimode core refractive index is between 0.04 to 0.06 for the low indexed germania (Ge) doped silicate multimode fibers of this approach.
As is known, the terminology “fiber Bragg grating” refers to a grating in which incident light is reflected back along the same fiber by a “short period” (a.k.a. Bragg) grating in the fiber and the fabrication of gratings is known. Fiber Bragg gratings (FBGs) couple power from one mode to another provided that the propagation constants of the two modes satisfy the following grating equation:                                           β            1                    -                      β            2                          =                              2            ⁢                                                   ⁢            π                    Λ                                    Eq        .                                   ⁢                  (          1          )                    where β1 and β2 are the propagation constants of the two modes, Λ is the grating period in the fiber, and first order diffraction is assumed for simplicity. When a forward propagating mode reflects into the identical backwards propagating mode, the Bragg condition becomes λB=2neffΛ, where neff is the effective index of the mode (β=(2π/λ)neff) and lies between the core index ncore and the cladding index, nclad for guided modes (nclad<neff<ncore). Forward propagating modes may also reflect into other modes when mode orthogonality is no longer maintained, for example when UV induced index changes due to the FBG itself perturb the index profile sufficiently. The index profile needed depends on fiber geometry, cladding material, and the exact wavelengths for the particular application.
As with the double-clad fiber laser, to enable the maximum launch of optical power from the high power pump source into the laser cavity of either the double-clad fiber or the two-stage multimode to single-mode fiber laser, the optical cavity needs to have a large numerical aperture (NA) which is related to the index contrast. However, an increased index delta for proving power enhancement requires more design, testing, and manufacturing complexities to be first solved.
Air-clad fibers are known. But to date, a mass-manufacturable air-clad three-level lasing fiber laser is not known.
Therefore there is a continued need to increase the power output of a fiber laser, whether double-clad or two-staged, while increasing the reliability and simplifying the packaging and manufacturing of the fiber laser, which will also reduce the cost of the fiber laser.